Manual

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7.4 Using hedgehog.log file
The system automatically records all measured positions in the hedgehog.log
file that is stored in the same folder as the Dashboard.exe file
The data is written in csv format; each line describes the position of one of the
hedgehogs at a certain moment
The line format is described here
7.5 System accuracy evaluation
1) Accuracy of distances measurement.
Marvelmind navigation system can measure distances between beacons with
accuracy of +/- 2cm if it uses correct ultrasound speed in measurements
The ultrasound speed depends of many factors: temperature of air, pressure,
humidity and so on
The main factor is temperature. In temperature range of -20…+50 °C the speed
of ultrasound changes on about 0.6 m/ (s* °C). It gives distance error about (0.6
/ 340) *100% ~ 0.17%/ °C. So caused by incorrect temperature setting absolute
error of distance measurement is 0.17% of real distance between beacons. For
example, with distance 30 meters and 5 °C error, this gives 0.85%*30 ~ 0.25
meters’ error. Marvelmind system allows to setup temperature of air in the system
settings
2) Accuracy of position measurement.
Marvelmind system uses trilateration algorithm to calculate position by distances.
The inaccuracy of position calculation is related to inaccuracy of distances
measurement and to geometry of relative location of stationary and mobile
beacons.
Basic trilateration formulas are given in this article:
https://en.wikipedia.org/wiki/Trilateration
As you see, the position of mobile beacons X, Y, Z is calculated from positions of
3 stationary beacons which are set by values of d, i, j. One of the beacons was
shifted to (0,0) position to simplify formulas in the article. In formulas for X, Y we
see d and j in denominators. This means that with low values of d and j small
error of this value can cause large position error.
Please see the picture of the beacons in the article - in more simple words, in
means that if one of three beacons is close to line connecting other two beacons,
it gives increased inaccuracy of locating mobile beacon.
For example, assume d= 10, i= 5, j= 0.1, r1= 7, r2= 7, r3= 4.8.
We get x= 5, y= 2.4375, z = 4.25.
If we suppose that j=0.101 (0.1 cm error), we receive x= 5, y= -0.06, z= 4.89.
You see very large Y error.
Another example for Z. Assume mobile beacon is relative close to plane of
stationary beacons:
d= 8, i= 4, j= 6, r1= 5.02, r2= 5.02, r3= 3.01.
This gives X=4, Y= 3.01169, Z= 0.36.
If we suppose r3= 3.0 (1 cm error), we receive X=4, Y= 3.016, Z= 0.44. Error on
Z is about 8 cm.
Also, with r1= 5, r2= 5, r3= 3, Z will be 0. As you see, low change of distances
causes large change of Z value near the plane